This invention relates to a direct sequence CDMA receiver and, more particularly, for demodulating using one sample per chip to reduce receiver complexity.
Wireless communications has been expanding at a phenomenal rate, as more radio spectrum becomes available for commercial use and as cellular phones become commonplace. Technology is currently evolving from analog communications to digital communications. Speech is represented by a series of bits. The bits are modulated and transmitted between a base station and a mobile station. Each of the base station and mobile station has a transmitter and a receiver. The receiver demodulates the received wave form to recover the bits, which are then converted back into speech. There is also a growing demand for data services, such as e-mail and Internet access, which require digital communications.
There are numerous types of digital communication systems presently available. Frequency-division multiple access (FDMA) divides the spectrum into plurality of radio channels corresponding to different carrier frequencies. These carrier frequencies may be further divided into time slots, referred to as time-division multiple access (TDMA) as is known in the D-AMPS, PDC and GSM digital cellular systems. Alternatively, if the radio channel is wide enough, then multiple users can use the same channel using spread spectrum techniques and code-division multiple access (CDMA).
Direct sequence (DS) spread spectrum modulation is commonly used in CDMA systems. Each information symbol is represented by a number of xe2x80x9cchipsxe2x80x9d. Representing one symbol by many chips gives rise to xe2x80x9cspreadingxe2x80x9d, as the latter typically requires more bandwidth to transmit. The sequence of chips is referred to as the spreading code or signature sequence. The code has a chip rate that is higher than the bit rate of the information signal. At the receiver, the received signal is despread using a despreading code, which is typically the conjugate of the spreading code. IS-95 and J-STD-008 are examples of DS-CDMA standards.
Coherent rake reception is commonly used with coherent DS-CDMA systems. The received signal is despread by correlating to the chip sequence, and the spread value is weighted by the conjugate of a channel coefficient estimate, removing the phase rotation of the channel and weighting the amplitude to indicate a softer confidence value. When multi-path propagation is present, the amplitude can vary dramatically. Multi-path propagation can also lead to time dispersion, which causes multiple, resolvable echoes of the signal to be received. Correlators are aligned with the different echos. Once the despread values have been weighted, then they are summed. This weighting and summing operation is commonly referred to as rake combining.
Modulation schemes which allow non-coherent demodulation may be used in DS-CDMA systems. For example, differentially encoded phase may be used, such as DPSK, allowing differential detection at the receiver. Also, as in the IS-95 uplink, M""ary orthogonal modulation may be used, allowing non-coherent detection at the receiver. The advantage of non-coherent detection is that channel estimation is not needed. Channel estimation may be difficult due to how fast the channel changes or how noisy the signal is. With non-coherent detection, rake combining of detection values corresponding to different echoes can still be performed to gain path diversity. Combining signals from different antennas is also possible.
Referring to FIG. 1, a digital communications systems 10 is illustrated. Digital symbols are provided to a transmitter 12. The transmitter 12 maps the symbols into a representation appropriate for the transmission medium or channel 16, such as a radio channel, and couples the signals to the transmission medium via an antenna 14. The transmitted signal passes through the channel 16 and is received at an antenna 18. The received signal is passed to a receiver 20. The receiver 20 uses a radio processor 22, a baseband processor 24, and a post-processing unit 26.
The radio processor 22 tunes to the desired band and desired carrier frequency and amplifies, mixes and filters the received signal down to baseband. The signal is sampled and quantized, ultimately providing a sequence of baseband received samples. Since the original radio signal has in-phase (I) and quadrature (Q) components, the baseband samples typically have I and Q components, giving rise to complex baseband samples. The baseband processor 24 detects the digital signals that were transmitted. It may also produce soft information, which gives information regarding the likelihood of the detected symbol values. The post processing unit 26 performs functions that depend on the particular communications application. For example, it may use the soft detected values to perform forward error correction decoding or error detection decoding. It may convert digital symbols into speech using a speech decoder.
The performance of DS-CDMA systems is limited by interference from other users. The despreading operation provides some degree of interference suppression, allowing multiple users to overlap in time and frequency. However, the capacity is limited. To improve receiver performance, interference cancellation has been used. One approach is successive cancellation of interference in which users are detected and subtracted in signal strength order, starting with the strongest user. Ideally, subtraction is based on the user""s symbol value and channel response information, referred to as coherent successive cancellation. In practice, the symbol value may be unreliable and the channel response may be unknown. A form of non-coherent cancellation can be used, as described in U.S. Pat. No. 5,151,919, assigned to the assignee of the present application, the specification of which is incorporated by reference herein. In this patent the despread value is used for signal subtraction rather than channel information.
With reference to FIG. 2, a prior art method of non-coherent successive cancellation is illustrated. The method begins at a block 32. At a block 34, user signals are ordered by signal strength. Signals may be ordered as described in U.S. Pat. No. 5,151,919. For each user, starting with the strongest, despreading is performed to obtain correlation value using normal spreading waveforms at a block 36. These correlation values are used to detect the information symbols at block 38. The correlation values are respread using the normal spreading waveform at block 40, and the respread signal is subtracted from the composite received signal at a block 42. A decision block 44 determines whether more users need to be demodulated. If so, then the processing returns to the block 36. If not, the routine ends at a block 46.
While non-coherent successive cancellation improves performance, there is a relatively high error floor when plotting performance versus signal-to-noise ratio. This factor is analyzed in P. Patel and J. Holtzman, xe2x80x9cAnalysis of A Simple Successive Interference Cancellation Scheme In A DS/CDMA System,xe2x80x9d IEEEJ SeL Areas Commun., vol. 12, pp. 796-807, June 1994. This shows that interference from previously canceled users is not entirely eliminated.
The performance of non-coherent successive cancellation can be improved by accounting for how each cancellation step affects the remaining signals. One approach is to use signal orthogonalization, which is described in U.S. Pat. No. 5,615,209, assigned to the assignee of the above application, which is incorporated by reference herein. This approach is initially described for signals which are synchronized in time. Orthogonalization is performed using the spreading sequences of the different users. In one embodiment, the method in U.S. Pat. No. 5,615,209 is similar to that shown in FIG. 2 herein, except that respreading uses a modified sequence instead of the normal sequence. The modified sequence is obtained by applying the Gram-Schmidt orthogonalization process to the user""s spreading sequences. In another embodiment the steps of respreading and subtracting are eliminated. Instead, despreading is performed using a modified sequence rather than the normal sequence. This embodiment is illustrated in the flow chart of FIG. 3 herein. The method starts at a block 50. At a block 52, users are ordered by signal strength. For each user, starting with the strongest, the user""s spreading waveform is orthogonalized with respect to previous user""s spreading waveforms to produce a modified spreading waveform at a block 54. The modified spreading waveform may be obtained from the Gram-Schmidt process as described in U.S. Pat. No. 5,615,209. The modified spreading waveform is used at a block 56 to despread the user of interest, producing correlation values. These correlation values are used to detect information symbols at a block 58. A decision block 60 determines if there are more users to be demodulated. If so, then control returns to the block 54. If not, then the process ends at a block 62.
U.S. Pat. No. 5,615,209 also discusses orthogonalization for asynchronous signals. An example of asynchronous users is illustrated in FIG. 4. For asynchronous signals, the formation of a hybrid sequence is formed for use in orthogonalization. For the example in FIG. 4, user B""s spreading sequence for symbol 1 is orthogonalized with a hybrid sequence that is a combination of part of user 1""s detected symbol 1 and part of user 1""s detected symbol 2, such that the hybrid sequence is aligned with user 2""s symbol 1. With asynchronous signals, the time of arrival for each signal is different. The difference in arrival times can be fractions of a chip period. As a result, it is common to sample the signal multiple times per chip period so that one of the sampling instances corresponds to the center of a chip for a given signal. When using the method in U.S. Pat. No. 5,615,209 with multiple samples per chip, each signature sequence is a sampled version of the spreading waveform used at the transmitter. However, orthogonalization using multiple samples per chip increases complexity significantly.
The present invention is directed to overcoming one or more of the problems discussed above in a novel and simple manner.
In accordance with the invention, non-coherent and coherent successive cancellation are performed efficiently by canceling interference using different chip timing values. This allows each signal to be processed with only one sample per chip, reducing receiver complexity and battery or power supply drain.
In one aspect of the invention, there is provided a direct sequence CDMA receiver including means for receiving a sampled signal. Means are provided for sub-sampling the sampled signal in accordance with timing information to produce a chip-sampled signal. Means generate equivalent codes using the timing information and information about user spreading codes. Means are provided for processing the equivalent codes to produce orthogonalized codes and means correlate the chip sampled signal with the orthogonalized codes to produce despread values.
It is a feature of the invention that the generating means comprises means for interpolating user spreading codes.
It is another feature of the invention that the generating means comprises means for concatenating codes in different symbol periods using detected symbol values.
It is another feature of the invention that the processing means includes a Gram-Schmidt processor.
It is still another feature of the invention that the correlating means comprises means for correlating two component codes to produce component correlations and means for combining component correlations to produce the despread values.
It is still a further feature of the invention that the correlating means produces plural despread values and further comprising means for combining despread values to produce combined despread values.
It is an additional feature of the invention that the combining means corresponds to combining despread values from different signal echoes.
It is still a further feature of the invention that the combining means corresponds to combining the despread values from different antennas.
It is still an additional feature of the invention that the generating means generates equivalent codes corresponding to different echoes of a user""s signal.
There is disclosed in accordance with another aspect of the invention a direct sequence CDMA receiver including means for receiving a sampled signal. Means are provided for sub-sampling the sampled signal in accordance with timing information to produce a chip-sampled signal. Means store the chip-sampled signal to produce a buffered signal. Means correlate the buffered signal with spreading codes to produce despread values. Means are provided for generating equivalent codes using the timing information and information about user spreading codes. The equivalent codes are processed to produce orthogonalized codes. Means are provided for spreading the despread values using the orthogonalized codes to produce respread signals. Means are provided for subtracting the respread signals from the buffered signal to produce an updated buffered signal.
There is disclosed in accordance with yet another aspect of the invention the method of detecting transmitted symbols in a direct sequenced CDMA receiver comprising the steps of receiving a sampled signal; sub-sampling the sampled signal in accordance with timing information to produce a chip-sampled signal; generating equivalent codes using the timing information and information about user spreading codes; processing the equivalent codes to produce orthogonalized codes; and correlating the chip sampled signal with the orthogonalized codes to produce despread values.
Further features and advantages of the invention will be readily apparent from the specification and from the drawings.